Mycoplasmas are small prokaryotic organisms (0.2 to 0.3 μm) belonging to the class Mollicutes, whose members lack a cell wall and have a small genome size. The mollicutes include at least 100 species of Mycoplasma, 13 of which are known to infect humans. One of these species, M. pneumoniae, is a major cause of community-acquired pneumonia (non-pneumococcal bacterial pneumonia), a type of pneumonia which is responsible for up to 2 million respiratory tract infections in the United States annually. The incidence of mycoplasma non-pneumonic respiratory infection has been estimated to be 10 to 20 times higher than this number. See 2 GERALD L. MANDELL ET AL., PRINCIPLES AND PRACTICE OF INFECTIOUS DISEASES § D (5th ed. 2000); see also PATRICK R. MURRAY ET AL., MANUAL OF CLINICAL MICROBIOLOGY § V (6th ed. 1995).
While M. pneumoniae infections are generally mild, as many as 10% of those infected with the organism progress to bronchopneumonia, requiring treatment or hospitalization. The most common early presentation of an M. pneumoniae infection is tracheobronchitis. Radiological findings vary widely and often indicate a more severe condition than symptoms typically associated with an M. pneumoniae infection (e.g., flu-like dry cough, pharyngitis, fever, malaise, headache and nasal congestion) would suggest Recovery of the organism from extra-pulmonary sites is rare, although hematologic, musculoskeletal, cardiovascular, dermatologic and neurologic complications have been reported. See ELMER W. KONEMAN, COLOR ATLAS AND TEXTBOOK OF DIAGNOSTIC MICROBIOLOGY 862 (5th ed. 1997).
M. pneumoniae infections occur year round, especially in large populations, but typically peak in late fall or winter. The incidence of isolation of the organism increases with age, and it is second only to Streplococcus pneumoniae as a cause of pneumonia in the elderly. M. pneumoniae-related pneumonia is seen most commonly in children over 5 years of age and in young adults. Crowded military populations, camps and schools are particularly at risk for community-acquired pneumonia caused by M. pneumoniae infections. In one study, over 5% of pneumonias seen in young military recruits were associated with M. pneumoniae. See Gray et al., “Respiratory diseases among U.S. military personnel: countering emerging threats,” Emerg. Infect. Dis., 3:379–387 (1999).
The incubation period for M. pneumoniae, which typically ranges from 2 to 3 weeks, is significantly longer than most viral respiratory infections. After clinical manifestation, symptoms associated with a M. pneumonia infection generally last from 3 to 10 days. Appropriate antibiotic treatments can significantly shorten the duration of the respiratory symptoms associated with a M. pneumoniae infection. However, since culture and many available diagnostic tests are difficult, time consuming and/or not readily available, ineffective antibiotic treatments are often prescribed, thereby unnecessarily prolonging the symptomatic period.
Diagnosis of a M. pneumoniae infection is often based solely on clinical signs and symptoms. Although culture remains the gold standard for diagnosing a M. pneumoniae infection, isolation, detection and identification of the fastidious M. pneumoniae organisms is difficult and can take weeks to complete. Diagnosis has also been based on demonstrating the presence of cold agglutinins. However, this test is a nonspecific indicator, as cold agglutinins may never develop in some patients infected with M. pneumoniae and have also been observed with lymphoma and a variety of viral diseases, including mononucleosis caused by Epstein-Barr virus and cytomegalovirus. Assaying for complement-fixing antibodies has also been used to confirm infection with M. pneumoniae, but is of little practical value in guiding diagnostic and therapeutic decisions, as the antibodies arise too late in the infection. An enzyme immunoassay has also been developed for detecting IgM and IgG directed against M. pneumoniae, but is limited in its usefulness since it does not become positive until 1 to 2 weeks into infection. In addition, an antigen-capture, indirect immunoassay has been used to detect M. pneumoniae antigens in sputum samples; however, the reagents of this assay cross-react with M. genitalium antigens. See, e.g., Bartlett et al., “Community-Acquired Pneumonia in Adults: Guidelines for Management,” Clin. Infect. Dis., 26:811–838 (1998). Thus, a need exists for a sensitive and specific assay which can be used to determine the presence of M. pneumoniae in a test sample during a clinically relevant period.
Also of clinical relevance is the detection of M. genitalium in a test sample. M. genitalium, which is thought to be a cause of nongonococcal urethritis (NGU), a sexually transmitted disease, has been detected to a significantly greater extent in symptomatic males than in asymptomatic males. See Yoshida et al., “Phylogeny-Based Rapid Identification of Mycoplasma and Ureaplasmas from Urethritis Patients,” J. Clin. Microbiol., 40:105–110 (2002). In addition to NGU, M. genitalium is thought to be involved in pelvic inflammatory disease, which can lead to infertility in women in severe cases. See JACK MANILOFF ET AL., MYCOPLASMAS: MOLECULAR BIOLOGY AND PATHOGENESIS 417 (ASM 1992). M. genitalium may also cause disease in the respiratory tract, making it important for some assays to distinguish between the presence of M. pneumoniae and M. gentialium. See LEE H. HILBORNE ET AL., A REVIEW OF THE SCIENTIFIC LITERATURE AS IT PERTAINS TO THE GULF WAR ILLNESSES, VOL.1: INFECTIOUS DISEASES CH. 3 (Rand 2000). Therefore, it would be of clinical importance to have an assay for specifically detecting the presence of M. genitalium in a test sample which is capable of distinguishing between M. genitalium and M. pneumoniae. 